1. Field of the Invention
The present invention relates generally to a predistortion (PD) apparatus and method for the linearization of a power amplifier at the transmitter of a wireless communication system and, more particularly, to a low-cost digital predistortion apparatus and method using envelope detection feedback, in which a feedback method required for a transmitter having a predistortion function has been simplified into an envelope detection method, thereby providing a predistortion function.
2. Description of the Related Art
Power amplifiers are essential to communication systems, and generally have nonlinear characteristics. Spectral regrowth (or spectral broadening) that causes interference between adjacent channels and inband distortion that causes a reduction in bit error rate (BER) performance at a receiving end, chiefly result from the nonlinearity of a power amplifier.
In order to compensate for such nonlinearity, various linearization techniques, such as RF feed-forward, RF feedback, RF/IF pre-distortion and post-distortion have been proposed. Of these techniques, a digital predistortion (DPD) technique has been recognized as a powerful method suitable for recent wireless communication systems that transmit wideband signals with high peak-to-average power ratio.
Digital predistortion is a technique that implements a predistorter having the inverse characteristics of a power amplifier in a baseband in a digital manner and makes a signal, inversely distorted via the predistorter, have linear characteristics while passing through the nonlinear characteristics of the power amplifier.
FIG. 1 is a block diagram illustrating a transmitter having a conventional predistortion function. This conventional predistortion technique requires a feedback loop for converting the output signal of an amplifier into a digital baseband signal in order to estimate the nonlinear distortion characteristics of the power amplifier and calculates a predistortion parameter that is used to compensate for the characteristics. In this case, a demodulation function is implemented using a mixer, an analog filter, etc.
Accordingly, this technology is disadvantageous in that the complexity and cost of implementation of a transmitter having a predistortion function are very high and it is difficult to apply the technology to a system having a spatial limitation, such as a terminal.
Korean Unexamined Patent Application No. 10-2012-0070144 discloses a digital predistortion power amplification apparatus and a digital synchronization control method for the apparatus.
This digital predistortion power amplification apparatus includes a power amplifier configured to amplify and output an input signal to be wirelessly transmitted via an antenna, a bias adjuster configured to change the voltage of a bias signal based on the size of the input signal and to output the changed bias signal, and a digital predistorter configured to control the delays of the input signal of the power amplifier and the bias signal in a digital manner and to thereby control the synchronization between the input signal and the bias signal.
A predistortion technique is configured to compensate for the nonlinear characteristics of a power amplifier at the input terminal of the amplifier using inverse nonlinear characteristics and to thereby make the output of the power amplifier have linear characteristics, thereby maximizing the efficiency of the power amplifier.
In the implementation of a predistortion technique, it is necessary to feed back the output of a power amplifier, convert the output into a baseband signal and detect the baseband signal. However, in order to convert an RF signal into a baseband signal, a frequency mixer and an analog filter are added to a transmitter and thus disadvantages arise in that the structure of the transmitter is complicated and the volume thereof is increased.
Accordingly, a disadvantage arises in that, although predistortion can be easily implemented in a system having a low spatial limitation, such as a base station, it is not easy to apply the predistortion technique to a system having a high spatial limitation, such as a terminal.